Topic: Low mileage 750, or used 836?

[adams77]

I have a delima,  I have 2 engines, one is a low mileage 750 that looks great on the inside.  The other is an 836 that looks like it was used hard, the pistons are still useable, but they are just at .002" out of round. 

The 836 came with an Andrews cam that is in tip top shape (Im going to use the cam with either)

So the question is,  Should I plop the cam in the low mileage 750 or pull the top off of the 750 and use the 836 top end?

[MRieck]

Is the 836 head ported? If you put the 836 head on the stock engine check your valve to piston clearance carefully. If those pistons are .002 out forget them.

[Geeto67]

out of round pistons are basically junk, you will never get good ring sealing in a perfectly round cylinder. If the 836 engine is whole and running and you still have good ring seal, then use it till it blows and then put the low mielage 750 engine in the frame. If they are just pistons without an engine then buy some new ones if you want an 836. As for the andrews cam in the 750, my personal expirence is that stock cb750s don;t run well with anything other than a stock cam. The stock engine really can't take advantage of the cam like a ported or bored out engine can.

what year and model are the 750s? (eg. F, K, A?) (1969-1978) the heads are not always interchangable with other models of cb750 and they also differ from year to year.

[volz1fsu]

out of round pistons are basically junk, you will never get good ring sealing in a perfectly round cylinder.

Not exactly true, The pistons are at the maximum out of round but still usable.  The pistons have nothing to do with ring sealing, that is between the rings and the cylinders.  The maximum out of round for a cylinder is also .002" so that means with new rings, sealing is still possible.  I would not run a cylinder with out of round but I have access to bore engines at my disposal so cost is not an issue. 
I have seen many people run engines with some cylinder out of round and some taper but they were still in specs and they ran fine.  Why bore an engine when it is not necessary.  In your case it is just the pistons out of round and not the cylinder which is even more likely to be fine.  Make sure the cylinder is straight and proper piston clearance is not exceeded.  Get them honed with a nice crosshatch pattern (60 degrees) and a new set of rings.
The cam would be fine in the stock engine if you decided to go that route though.  It would have less low end  power than stock but great high end power because the engine would be able to breath better than stock and that is always a plus.

[cd811]

out of round? or out of spec?
piston measurements? or piston to cylinder clearance?
 

[adams77]

What I meant to say was "Taper"  I was at a loss for words the other day.  Not out of round.

The cylinder walls look great on the 836, but I did have a bit of resistance when I was removing the compression rings from the pistons (IS this normal?)

I would really like to use all the HI-PO parts on the build, but not if they are worn out.

[MRieck]

but I did have a bit of resistance when I was removing the compression rings from the pistons (IS this normal?)

It depends. Alot of carbon can cause the ring to stick or detonation can close the ring land and cause the top ring to stick. I have seen pistons run with nitrous have the lands close up. Clean everything well and measure your ring to land clearance with feeler gauges.

[adams77]

Clean everything well and measure your ring to land clearance with feeler gauges.

Thats great, I think I will go ahead with the 836 top end.  Speaking of cleaning, I have a glass bead blasting cabinet, could I bead blast the pistons to clean them without causing any damage?  And once they are blasted should I polish them or leave them with the dull finish?

I know there are mixed opinions on polishing heads does the same argument hold for the pistons?

[MRieck]

Clean everything well and measure your ring to land clearance with feeler gauges.

Thats great, I think I will go ahead with the 836 top end.  Speaking of cleaning, I have a glass bead blasting cabinet, could I bead blast the pistons to clean them without causing any damage?  And once they are blasted should I polish them or leave them with the dull finish?

I know there are mixed opinions on polishing heads does the same argument hold for the pistons?

Tape the ring lands over with a couple of wraps of masking tape- you want to keep those inner land surfaces smooth. I have used carb cleaner with a short,stiff natural bristle brush to get the lands clean. Bead blast the rest if the piston inside and out. Polishing the crown (only) is a good idea as it helps hold heat in (heat=power) and helps to keep carbon from sticking to it.